tokfandomcom-20200215-history
Basement rocks
, basement rock, }} In , basement and crystalline basement are the rocks below a or cover, or more generally any rock below s or s that are or in origin. In the same way, the s or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover". Supracrustal rocks (Supra (Latin for "above")) are rocks that were deposited on the existing basement rocks of the crust, hence the name. They may be further metamorphosed from both sedimentary and volcanic rocks. Continental crust Basement rock is the thick foundation of ancient, and oldest, and that forms the of s, often in the form of . Basement rock is contrasted to overlying s which are laid down on top of the basement rocks after the continent was formed, such as and . The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 3 miles thick. The basement rock of the crust can be 20–30 miles thick, or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface. Basement rock is visible, for example, at the bottom of the , consisting of 1.7-2 billion year old ( ) and ( ). The Vishnu Schist is believed to be highly metamorphosed igneous rocks and , from , and laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well. Age The basement rocks of the continental crust tend to be much older than the oceanic crust. The oceanic crust can be from 0-250 million years in age, and is usually thinner (10 miles or so) and composed of ic rocks. Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at and areas. Complexity The basement rocks are often highly metamorphosed and complex. They may consist of many different types of rock - volcanic, intrusive igneous and metamorphic. They may also contain fragments of that became wedged between plates when a was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form underplating, where the new igneous rock forms a layer on the underside of the crust. It is said that the majority of continental crust on the planet is around 1-3 billion years old, and it is theorised that there was at least one period of rapid expansion and accretion to the continents during the Precambrian. Cratons of the world (USGS) | Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma |} ]] Many continents can consist of several continental s - blocks of crust built around an initial original core of continents - that gradually grew and expanded as additional newly created terranes were added to their edges. For instance, consisted of most of the Earth's continents being accreted into one giant . Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents. A craton is an old and stable part of the , which consists of the Earth's two topmost layers, the and the uppermost . Having often survived cycles of merging and ing of s, cratons are generally found in the interiors of . They are characteristically composed of ancient , which may be covered by younger . They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into the Earth's . The term craton is used to distinguish the stable portion of the from regions that are more geologically active and unstable. Cratons can be described as , in which the basement rock crops out at the surface, and , in which the basement is overlaid by sediments and sedimentary rock. Cratons have thick lithospheric roots. Mantle shows that cratons are underlain by anomalously cold mantle corresponding to more than twice the typical thickness of mature oceanic or non-cratonic, . At that depth, craton roots extend into the . Craton lithosphere is distinctly different from because cratons have a neutral or positive , and a low intrinsic density. This low density offsets density increases due to and prevents the craton from sinking into the deep mantle. Cratonic lithosphere is much older than oceanic lithosphere—up to 4 billion years versus 180 million years. Rock fragments ( ) carried up from the mantle by magmas containing have been delivered to the surface as s in pipes called s. These inclusions have densities consistent with craton composition and are composed of mantle material residual from high degrees of partial melt. Peridotite is strongly influenced by the inclusion of moisture. Craton peridotite moisture content is unusually low, which leads to much greater strength. It also contains high percentages of low-weight magnesium instead of higher-weight calcium and iron. Peridotites are important for understanding the deep composition and origin of cratons because peridotite nodules are pieces of mantle rock modified by partial melting. peridotites represent the crystalline residues after extraction of melts of compositions like and . Usage In , the basement generally refers to rocks older than the . On top of this older basement s and s were deposited. The evaporites formed a zone on which the harder (stronger) limestone cover was able to move over the hard basement, making the distinction between basement and cover even more pronounced. In geology the basement refers to the , and early ( to ) rock units as the basement to the late Mesozoic and Andean sequences developed following the onset of along the western margin of the . When discussing the of the basement include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero s respectively. The term basement is used mostly in disciplines of geology like , and in which the (typically ) crystalline basement is not of interest as it rarely contains or . The term economic basement is also used to describe the deeper parts of a cover sequence that are of no economic interest. References Category:Earth